Voltage drop estimate

Estimate voltage drop for single phase and three phase circuits from conductor size, material, length and current.

What this means

Long runs lose voltage in the wire. Too much drop can cause motors to run hot and controls to act strangely. This is an estimate using a simple K factor; actual drop depends on conductor temperature, conduit and power factor.

Formula used

  • Single phase: VD = 2 × K × I × L ÷ CM
  • Three phase: VD = √3 × K × I × L ÷ CM
  • K ≈ 12.9 copper, 21.2 aluminum (Ω·cmil/ft); L = one way length in feet
  • Metric: CM = mm² × 1973.5

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